FIELD OF THE INVENTION
The present invention relates to a toner for use in an image forming method, such as an electrophotography, an electrostatic printing method or a magnetic recording method to visualize an electrostatic latent image and relates to a method of manufacturing a resin composition for use in the toner. More particularly, the present invention relates to a toner for developing an electrostatic image for use in a fixing method of a type for fixing a visual image formed with toner to a recording medium using heat and relates to a method of manufacturing a resin composition for use in the toner.
A variety of electrophotography methods is known as disclosed in U.S. patent application Ser. No. 2,297,691, Japanese Patent Publication No. 42-23910 and Japanese Patent Publication No. 42-24748. In general, a copied article is obtained by a method comprising the steps of: using photoconductive substances; forming an electric latent image on a photosensitive medium by any one of a variety of means; the latent image is formed by using toner; if necessary the toner image is transferred to a transferring medium, such as paper; and the developed image is fixed with heat, pressure, heat and pressure or vapor of a solvent. Toner left from transferring on the photosensitive member is cleaned and the foregoing process is repeated.
In recent years, there have been requirements for a copying machine of the foregoing type to reduce the size and weight, raise the copying speed and improve reliability. Also toner must have improved characteristics in the foregoing circumstance. A variety of methods and apparatuses for use in a process for fixing the toner image onto a sheet made of paper for example has been developed. Among others, the most general method is a heat fixing method using a hot roller. The hot roller fixing method is a method of fixing the toner image in such a manner that the sheet, on which a toner image to be fixed is formed thereon, is passed through the hot rollers under pressure while bringing the surfaces of the sheet contact with the surfaces of the hot rollers. Since the foregoing method is arranged in such a manner that the surfaces of the hot rollers and the toner image to be fixed and formed on the sheet are brought into contact with each other under pressure, an excellent thermal efficiency can be realized when the toner image is fixed on to the sheet. Therefore, the fixation process can quickly be completed, and therefore it is very effective for a high speed electrophotographic copying machine to employ the foregoing method.
However, the conventional hot roller fixation method encounters problems to be solved.
(1) A somewhat long waiting time takes place in which the formation of the image is inhibited until the hot roller is heated to predetermined temperature. PA1 (2) In order to prevent defective fixation occurring due to change of the temperature of the hot roller caused from passing of the recording medium or an external disturbance and to prevent transference of toner (so-called an "offset phenomenon") to the hot roller, the hot roller must be maintained at the optimum temperature. Therefore, the hot roller or the heating unit must have a large thermal capacity. This, however, leads to problems in that a large electric power is required and the temperature in the image forming apparatus is undesirably raised. PA1 (3) Since the rollers are heated considerably, the recording medium and the toner placed on the recording medium are cooled slowly when the recording medium is passed through the hot roller to be discharged. Therefore, the toner is caused to have high viscosity, and therefore the recording medium can be undesirably introduced into the roller portion, causing a risk to arise in that paper jamming takes place. PA1 (1) A method using a toner binder resin having two peaks in the molecular weight distribution; PA1 (2) A method characterized in that a polyolefin polymer having a low molecular weight typified by wax having a low molecular weight is added to the toner; and PA1 (3) A method characterized in that wax or the like is previously added to the binder.
Japanese Patent Application Laid-Open No. 63-313182 (corresponding to U.S. patent Ser. No. 5,149,941) discloses an image forming apparatus exhibiting short waiting time and small electric power consumption realized by a fixing unit arranged in such a manner that a visible toner image is, while interposing a heatproof sheet, heated employing pulse-like electric power. Similarly, Japanese Patent Application Laid-Open No. 1-187582 (corresponding to U.S. patent Ser. No. 5,149,941) discloses a fixing apparatus of a type for heating and fixing a visible toner image on to a recording medium while interposing a heatproof sheet, the disclosed apparatus being characterized in that the heatproof sheet has a heat resisting layer and a separation layer or a low-resistance layer so that the offset phenomenon is effectively prevented.
In order to realize a fixing method exhibiting excellent fixation of the visible toner image on a recording medium, capable of preventing the offset phenomenon, shortening the waiting time and reducing the electric power consumption, the toner must have desired characteristics as well as the foregoing fixing apparatus.
Hitherto, toners have been provided with excellent fixation and offset resisting characteristics by the following methods:
The foregoing method (1) has been disclosed, for example, in Japanese Patent Application Laid-Open No. 56-16144 (corresponding U.S. Pat. No. 4,499,168), Japanese Patent Application Laid-Open No. 2-235069, Japanese Patent Application Laid-Open No. 63-127254 and Japanese Patent Application Laid-Open No. 3-26831. The method (2) has been disclosed in, for example, Japanese Patent Publication No. 52-3304 (corresponding U.K. Patent No. 1,442,835), Japanese Patent Publication No. 52-3305 (corresponding U.K. Patent No. 1,442,835), Japanese Patent Application Laid-Open No. 57-52574, Japanese Patent Application Laid-Open No. 58-215659, Japanese Patent Application Laid-Open No. 60-217366, Japanese Patent Application Laid-Open No. 60-252361 and Japanese Patent Application Laid-Open No. 60-252362.
Although the method using the binder resin having two peaks in the molecular weight distribution therein and the method in which a releasing agent of a certain type is contained in the toner are able to somewhat improve the fixation and the offset resistance, binder components are sometimes nonuniformly dispersed. In this case, other components, for example, wax, cannot easily be dispersed or a specific component can easily be distributed eccentrically or freed. As a result, image contamination takes place due to fog or undesirable fusion to the photosensitive member or filming take place. Another method has been disclosed in Japanese Patent Application Laid-Open No. 3-72505 in which the molecular weight of the peak having the high molecular weight is further enlarged. However, the foregoing method is unsatisfactory to further improve the offset resistance. The foregoing method of simply further enlarging the molecular weight sometimes inhibits the dispersion of the other components as described above.
If kneading conditions to be employed in a melting and kneading process in the toner manufacturing method are made severer to improve the compatibility and the dispersion characteristics of the components of the toner, the breakage of molecular chains of the binder resin occurring due to kneading decreases the molecular weight of the binder resin. In this case, a problem arises in that the offset resistance deteriorates, and in particular hot offset resistance at high temperatures deteriorates. If a large quantity of wax is added to obtain satisfactory offset prevention characteristics, blocking resistance deteriorates and the wax dispersion further deteriorates. As a result, practical problems take place in that the image quality deteriorates due to enhancement of contamination of the surface of the developer carrier such as the carrier or the sleeve.
The method (3) characterized in that the wax or the like is previously added to the binder resin has been disclosed in, for example, Japanese Patent Application Laid-Open No. 62-195683, Japanese Patent Application Laid-Open No. 3-185458, Japanese Patent Application Laid-Open No. 56-87051, Japanese Patent Application Laid-Open No. 2-2578, and Japanese Patent Application Laid-Open No. 2-12160.
As compared with the methods (1) and (2), the method (3) exhibits excellent dispersion if the toner is made of binder resin having a narrow distribution of the molecular weights, and accordingly the offset resistance can somewhat be improved. However, the distribution of the molecular weights in the binder resin must be widened to further preferably improve the fixation at low temperatures and improve the offset resistance. If the wide distribution is applied to the binder having the two peaks in the molecular weight distribution thereof, the components having low molecular weights and those having high molecular weights are further separated, causing the compatibility of the components of the two types to further deteriorate. Therefore, the effect obtainable from previously dissolving the wax component cannot be obtained. What is worse, the surface of the photosensitive member or that of the carrier of the developer can be damaged or the toner can be solidified and fixed. It was found that the foregoing tendency is enhanced in proportion to the weight average molecular weight of the sole polymer component in the binder (specifically, it is made enhanced when Mw.gtoreq.1,000,000). If a polymer component satisfying Mw.gtoreq.1,000,000 is used and the resin composition satisfies Mw/Mn&gt;30, desired fixation characteristics and the offset resistance cannot be realized. What is worse, a critical problem takes place in matching with the developer as described above.
However, the various characteristics, which are required for the toner to satisfy, cannot simultaneously be satisfied in spite of rising of a desire in recent years while improving the level of the characteristics. Although a collective study including the improvement in the developing characteristics has been made, the results have not been satisfactory.
The inventors of the present invention have investigated resin compositions, polymer components forming the resin composition and methods of manufacturing the compositions and the polymers, resulting in that toner is obtained which exhibits: (i) a considerably wide temperature range in which fixing can be performed, (ii) excellent reproducibility of fine lines and (iii) performance capable of forming stable images having excellent image quality.